This invention relates to methods and apparatus for proving fluid flow meters, such as gas meters and the like, and in particular to a method and apparatus for proving such fluid flow meters at low flow rates.
Gas meters presently employed by the gas utility industry are mechanical meters of the double bellows type. Such meters are proven using apparatus commonly referred to as a bell prover system. In such a system, a copper bell of accurate dimension is allowed to descend at a constant rate into a tank of light oil or water. As the bell descends, a suitable test fluid, typically air or natural gas, is passed through the meter under test. The volume of air or natural gas which is passed through the meter is determined by the amount of linear movement of the bell. The position of the bell accurately defines the volume of test fluid which has been passed through the meter under test.
Typically, in proving gas meters using bell prover systems, a source of air or natural gas is connected to the meter under test and the flow rate of the gas meter is adjusted by selecting a suitable orifice which is connected in series with the source of test fluid and the meter under test. With the flow rate of the gas meter set, the proof run is initiated. With gas meters presently available, initiation of a proof run is effected by interrupting a light source using the calibration dial of the meter. At the start of the proof run, the test fluid supply is switched rapidly to the bell. After a selected volume of the test fluid, typically 1, 2 or 5 cubic feet, has passed through the meter of the test as indicated by the calibration dial and the bell, the light source is interrupted because the calibration dial has registered one complete revolution. When the light source is again interrupted, the fluid outlet of the bell is closed off, terminating the supply of the test fluid to the meter.
The position of the bell is then accurately recorded electronically, yielding the exact amount of the test fluid that has passed through the meter under test during the time it recorded passage of one cubic foot of fluid as indicated by the calibration dial of the meter. From this measurement, the accuracy or proof of the meter can be calculated. The information obtained can be used to adjust the mechanical mechanism of a meter that fails the proof test. One shortcoming of this measurement arrangement is that at a typical very low flow rate, say 0.25 cubic foot per hour (cfh), it will take four hours to complete a one cubic foot proof test on a gas meter.
In the co-pending U.S. patent application Ser. No. 140,714 of Pearman et al, which is entitled ELECTRONIC GAS METER, and which is assigned to the assignee of this application, there is disclosed a gas meter which includes a solid state sensor and solid state signal processing circuits for measuring gas flow volume. This gas meter does not have a calibration dial available for controlling a proof test in a manner similar to mechanical meters of the double bellows type as described above. Also, the meter does not have a mechanical adjustment to improve its accuracy. Thus, proof test techniques and calibration adjustments heretofore used for mechanical gas meters cannot be used on electronic gas meters of this type.
Thus, it would be desirable to have a method and apparatus for proving a fluid flow meter of the type incorporating solid state sensing and signal processing circuits.
It would also be desirable to have a method and apparatus for proving a fluid flow meter at low flow rates.